A reset circuit is used in many complex logic and control circuits to establish a predefined logical state or condition into which the logic gates and other important functional elements are placed or forced when the circuit is first powered up or energized. If the logic gates and other functional elements do not commence operation from a predetermined state, the functionality of the circuit will be flawed to the point of making the circuit inoperable or unreliable. Without a reset circuit, the logic gates and other functional elements may start in various indeterminate, random and unknown states because these elements usually have slightly different operating characteristics which come into play when the elements are first powered up.
It is generally impossible or difficult to characterize or deal with those varying characteristics without using a reset circuit. Once the elements are powered up and operating, their functionality is reliable and predictable. Indeed it is this predictable functionality which allows such elements to be employed with reliability in complex control, logic and other circuits.
The typical reset circuit uses timing circuit elements which assert a reset signal until the operating voltage reaches a predetermined reliable operating level, at which point the predictable operation of the logic and control elements will prevail. When the reliable operating voltage level is reached, the reset signal is removed or de-asserted. Either the timing elements of the reset circuit can respond directly to the operating voltage level by asserting the reset signal until a capacitor is charged to the predetermined reliable operating voltage, as one example, or the timing elements can respond indirectly to the operating voltage by asserting the reset signal for a predetermined time after which it is assumed that the applied voltage has reached the operating level, as another example. In either case, the reset signal is removed or de-asserted when it is believed that a reliable operating level of voltage has been reached.
One well known type of reset circuit employs a combination of discrete diodes, resistors, and capacitors as the timing circuit elements. A reset circuit of this type consumes a relatively large amount of physical space, making it difficult or impossible to integrate with the other elements of the circuit which it controls. Another disadvantage is that the timing function may vary substantially with temperature, making the reset circuit variable in performance and reliability when used in environments of wide temperature variations. It is usually the characteristics of the capacitor which vary the most with the temperature variation.
Another type of well known reset circuit uses a counter to achieve the timing function. When the circuit is initially powered up, the counter starts incrementing to a selected count value. Until the counter reaches selected count value the reset signal is asserted. Once the selected count value is reached, the reset signal is de-asserted. A counter-type reset circuit offers the advantage of easier integration with the other circuit elements which it controls, because the counter is usually formed primarily by transistors which consume small amounts of space in an integrated circuit compared to the timing capacitor of the discrete component-type of reset circuit. However, the disadvantage to a counter-type reset circuit is that the counter itself is subject to the same problems as the circuit which it is intended to control. Upon initial power up, the counter will start from some random, indeterminate and unknown value, thereby also making the timing function, and hence the reset function, random, indeterminate and variable.
In addition to starting operation of a circuit from a known and predetermined state, a reset circuit also asserts the reset signal after momentary interruptions of electrical power. If the power interruption is of a very short duration, the circuit need not be reset if the voltage drop across the circuit elements is not great enough to cause the then existing states of the circuit elements to be altered. Conventional reset circuits may not distinguish between longer momentary power interruptions which require the circuit to be reset and shorter momentary power interruptions which do not require the circuit to be reset.
It is with respect to these considerations and other background information relative to reset circuits that the significant improvements of the present invention have evolved.